Roberts Bay Condition Report for 2014

Summary:

The overall health of Roberts Bay improved in 2014. Chlorophyll a concentration decreased significantly from 2013, bringing its mean below the target value. While the mean nitrogen concentration increased very slightly, it is still below the threshold level and the upward trend seen in 2013 has leveled out. Mean phosphorus concentration decreased and remains well below the target value of 230 μg/l.

Water Quality: Nitrogen and phosophorus retained their ratings from 2013 (“Good” and “Excellent”, respectively). Mean nitrogen concentration was 0.5205 mg/l and mean phosphorus concentration was 0.0949 mg/l. Chlorophyll a concentration (0.0067 mg/l) improved enough to boost its rating to “Excellent”. The mean for chlorophyll a was calculated as an arithmetic mean and the means for nitrogen and phosphorus were calculated as geometric means (per the Numeric Nutrient Criteria outlined in the Florida Administrative Code, section 62-302.532).

Biotic Indicator: Measurement of the biotic indicator, seagrass, was performed in 2014 by the Southwest Florida Water Management District. Total seagrass acreage in Roberts Bay increased slightly to 321 acres from its previous value and is creeping toward the desired target of 348 acres.

Water Chemistry Ratings

Total nitrogen, total phosphorus, and chlorophyll a levels are monitored carefully by
water resource managers and used by regulatory authorities to determine whether a bay
meets the water quality standards mandated by the Clean Water Act. The trend graphs for
these indicators are shown below, along with their target and threshold values. A target
value is a desirable goal to be attained, while a threshold is an undesirable level which
is to be avoided. Learn More about these ratings and how they are calculated »

The Five-year Trend Graphs below illustrate the general trend of water quality parameters. They show a six-month running average, which moderates high and low values in the data.

Chlorophyll a

The rating for Chlorophyll a is determined by comparing the annual arithmetic mean of its sampled values to fixed target and threshold values. The annual arithmetic mean is shown in the graph at left, as compared to its established target and threshold values.

Nitrogen, Total

The rating for Total Nitrogen is determined by comparing the annual geometric mean of its sampled values to target and threshold values. The target and threshold values are variable for Sarasota Bay, established annually, while they are fixed for the other bays. The geometric mean is found by multiplying together all the sample values for the calendar year and then taking the nth root of the result, where n is the number of samples. The annual geometric mean for Total Nitrogen is shown in the graph at left, as compared to its established target and threshold values.

Phosphorus, Total

The rating for Total Phosphorus is determined by comparing the annual geometric mean of its sampled values to fixed target and threshold values. The geometric mean is found by multiplying together all the sample values for the calendar year and then taking the nth root of the result, where n is the number of samples. The annual geometric mean for Total Phosphorus is shown in the graph at left, as compared to its established target and threshold values.

Other Measures of Bay Health

In addition to nutrient levels and chlorophyll concentration, dissolved oxygen levels, and water
clarity are also objective indicators of bay health. These have complex interactive cycles which
are affected by rainfall, temperature, and tidal action, as well as other factors. High nutrient
levels (nitrogen and phosphorus) can stimulate excessive growth of marine algae (indicated by
chlorophyll a level), resulting in reduced water clarity (and increased light attenuation) and
depleted oxygen levels. Both plants and animals in a bay need oxygen to survive, and the seagrasses
which provide food and cover for bay creatures need light for photosynthesis.

Bay Contour Maps (2014)

Contour mapping is one of the best ways to visualize spatial differences in coastal water quality.
The interactive map shown below presents monthly data for one selected water quality indicator atop
an aerial view of the bay. Choose a different water quality parameter from the list at the top to change the map.
Learn More about Water Quality Contour Mapping »

Visit the Water Quality Contour Mapping Tool to view and compare monthly water quality contour maps for ten different water quality indicators. In addition, you can generate your own custom maps.

Contour Legend:

Less than 1 mg/l

1.0 - 5.9 mg/l

6.0 - 10.9 mg/l

11.0 - 17.9 mg/l

Greater than 18 mg/l

Less than 300 ug/l

300 - 499 ug/l

500 - 699 ug/

700 - 899 ug/l

900 - 3,499 ug/l

Greater than 3,500 ug/l

Less than 100 ug/l

100 - 199 ug/l

200 - 299 ug/

300 - 399 ug/l

400 - 499 ug/l

Greater than 500 ug/l

Less than 4.0 ppt

4.0 - 10.0 ppt

10.0 - 18.0 ppt

18.0 - 35.0 ppt

Greater than 35.0 ppt

Seagrasses

Among the most important habitats in Florida's estuarine environments, seagrass beds are indispensable
for the role they play in cycling nutrients, supplying food for wildlife, stabilizing sediments, and
providing habitat for juvenile and adult finfish and shellfish. Use the interactive map below to
observe the size, density and location of seagrass beds from year to year. The graph shows how the total
amount of seagrass in the bay has changed over time.
Learn More about Seagrasses »

Showing Seagrass Coverage for:

1948

1999

2001

2004

2006

2008

2010

2012

2014

2016

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Land Use / Land Cover

Land use within a bay's watershed has a major effect on its water quality.
In general, less development means better water quality. Land Cover/Land Use
classifications categorize land in terms of its observed physical surface
characteristics (upland or wetland, e.g.), and also reflect the types of activity
that are taking place on it (agriculture, urban/built-up, utilities, etc.).
Florida uses as its standard a set of statewide classifications which were developed
by the Florida Department of Transportation.
Learn More about Land Use and Land Cover »

Roberts Bay is located within the Sarasota Bay Watershed. The chart below shows the land use / land cover characteristics for Sarasota Bay Watershed within the boundary of this Water Atlas. View details about the Sarasota Bay Watershed »